1. Field of the Invention
The following description relates to an optical pick-up and an optical information storage medium system using the optical pick-up.
2. Description of the Related Art
Optical information storage media including, for example, optical discs, are recorded and readout by optical recording/reproducing apparatuses. The optical recording/reproducing apparatuses may use laser beams having different wavelengths and objective lenses having different numerical apertures, according to the amount of information to be stored. In other words, as the capacity of an optical disc is increased, an objective lens with a light source having a shorter wavelength or with a greater numerical aperture may be used. For example, in a case of a compact disc (CD), a beam having a wavelength of about 780 nm and an objective lens having a numerical aperture of about 0.45 are used, in a case of a digital versatile disc (DVD), a beam having a wavelength of about 650 nm and an objective lens having a numerical aperture of about 0.6 are used, and in a case of a Blu-ray Disc (BD), a beam having a wavelength of about 405 nm and an objective lens having a numerical aperture of about 0.85 are used. The BD having a greater capacity than the DVD, which has a greater capacity than the CD.
As described above, in order to increase the density of an optical disc, a size of a light spot formed on the optical disc is decreased. In order to decrease the size of the light spot, a wavelength of a beam is decreased, and/or a numerical aperture of an objective lens is increased.
As another aspect, an expensive component is used to decrease the wavelength of the beam, and in response to the numerical aperture of the objective lens being increased, a focus depth is decreased by an amount corresponding to the square of the numerical aperture. A coma aberration is increased by an amount corresponding to the cube of the numerical aperture, so that an increase in the density of the optical disc, by decreasing the size of the light spot, may be limited.
Due to an increase in demand the optical disc with a larger capacity, a multilayer structure having more than one recording layer is used. Thus, the recording capacity of an optical disc having at least two recording layers formed on one side or both sides of the optical disc may be greater than the recording capacity of an optical disc having a single recording layer.
As described above, a multilayer optical disc may be used to increase a capacity of the optical recording/reproducing apparatuses. As another aspect, in response to the multilayer optical disc being used, light reflected from an adjacent layer other than a target reproducing/recording layer creates interference with signal light. The interference with signal light may create noise.
In general, a differential push-pull (DPP) method may correct an offset of a push-pull signal that is generated in response to an eccentricity disc being reproduced. The differential push-pull (DPP) method may be selected as a tracking method of a recordable optical disc. According to a general DPP method, a grating may be used to divide light into 0th-order light (main light) and two first-order lights (sub-lights). In consideration of an efficient use of light, a ratio of − first-order light: 0th-order light:+ first-order light with respect to the amounts of divided lights may be approximately 1:10:1.
In response to the DPP method being used to detect a tracking error signal in a dual-layer optical disc having two recording layers, 0th-order light reflected from an adjacent layer may overlap with first-order light reflected from a target reproducing/recording layer causing a deterioration of the tracking error signal. In other words, the amount of light of 0th-order light reflected from the target reproducing/recording layer is significantly different from the amount of light of the 0th-order light reflected from the adjacent layer. On the other hand, the amount of light of the first-order light reflected from the target reproducing/recording layer is not significantly different from the amount of light of the 0th-order light reflected from the adjacent layer. Thus, the 0th-order light of the adjacent layer significantly affects a differential signal (a sub push-pull (SPP) signal with respect to sub-light) used to detect the tracking error signal in the DPP method.
In order to prevent the SPP signal from being unstable due to interlayer interference light, a one-beam tracking method has been proposed. The one-beam tracking method does not use sub-lights but instead uses main light. As another aspect, in the one-beam tracking method, the amount of light of the signal light is great, but the amount of light of the signal light is not free of interlayer interference. In response to a multilayer optical disc being embodied, an interlayer gap may be further decreased. In addition, as the interlayer gap decreases, a push-pull detection signal with respect to main light, i.e., a main push-pull (MPP) signal may further be deteriorated.